Thermal desorption of Ba from tungsten

Scandate cathodes have demonstrated extremely high electron emission densities, and developments are underway to improve the emission uniformity and the replenishment of the active surface layer under ion bombardment. A thorough understanding of the emission mechanism is needed to guide the approach of the development. Much of the surface studies characterize the equilibrated films, and little understanding has been known about the dynamics of the interactions. Some insight into the desorption dynamics can be gained through temperature-programmed desorption studies in which a direct-line-of-sight quadrupole mass analyzer is employed to monitor a specific desorbing atomic or molecular species as a function of the substrate temperature. In this paper, we discuss the experimental method in applying the technique to study BarBaO on a tungsten substrate. Multiple adsorption states were observed and each appeared as a distinct desorption feature, from which one is able to determine separately for each state the binding energy, the desorption kinetics and the population. In ascending order of binding energy, we have observed the multilayer Ba adsorption state, Ba-on-W adsorption state, and oxygen-enhanced adsorption state. q1999 Elsevier Science B.V. All rights reserved.